The present invention relates to an arrangement for detecting coded information and more particularly to an arrangement in which the code is held on an active transponder. Such arrangements are useful in the identification of articles, or to control the entry of personnel to zones of limited access, for example.
One such arrangement is shown in FIG. 6 of Patent Application GB No. 2017454A. A circuit is provided with an encoder in the form of a data coding chip. The chip comprises a plurality of diodes, selected ones of which are blown to permit a unique serially coded pulse train to be produced. A disadvantage of this previous arrangement is that once the code has been selected it cannot be readily changed. However, the use of a chip containing all relevant circuitry is a very economical way of manufacturing a transponder in large numbers. It would be a considerable advantage if such a transponder could be produced in which the coding could be changed at will.
British patent specification No. 1 555 606, supported by its Parent Patent No. 1 538 157, describes a transponder in which a unique code is stored in a memory and can be changed. However, this system is on a very different scale since the transponder is to be mounted on vehicles, its weight is not critical, and its manufacture would be extremely expensive. It operates exclusively using high frequency electromagnetic signals (10 GHz) and waveguides. Such a device requires relatively high current levels for its operation, and therefore requires a larger battery or more frequent changes.